1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device, and more particularly to a nitride semiconductor light-emitting device having improved electrical and optical characteristics by improving crystallinity of an active layer.
2. Description of the Related Art
Recently, advent of a nitride semiconductor capable of emitting a blue-green light has lead to realization of three primary colors and white light in semiconductor light-emitting devices. Therefore, application areas of light-emitting devices made of such a nitride semiconductor are further extended and thus widely used in lightings in which high-directivity is required, such as back light sources of key pads and liquid crystal displays, traffic lights, guide lights of runways, and the like.
Generally, as a nitride semiconductor, semiconductor crystalline material having the formula of AlxInyGa(1−x−y)N wherein x is a number between 0 and 1, y is a number between 0 and 1, and the sum of x and y is a number between 0 and 1, is used. However, there in no commercially available substrate having the same crystal structure as that of the nitride semiconductor while exhibiting lattice matching therebetween, and thus a sapphire substrate, which is electrically insulative, is primarily used.
Consequently, there are crystal defects resulting from difference in a lattice constant and thermal expansion coefficient between the sapphire substrate and the nitride semiconductor layer grown thereon. In order to lower this problem, a buffer layer may be used on the sapphire substrate, but the buffer layer alone cannot completely resolve such a problem associated with crystal defects. In particular, in InGaN/GaN layer used as an active layer for generating/emitting a blue-green light, these crystal defects raise a problem of greatly reducing a light-gain.
In order to overcome such a problem of light-gain deterioration occurred in the active layer composed of InGaN/GaN, a conventional art has increased a carrier concentration by doping the active layer with a predetermined concentration of elemental Si or Ge. Generally, the active layer has a structure having a quantum barrier layer composed of undoped AlGaN or GaN and a quantum well layer composed of undoped InGaN alternatively laminated. At least one layer among them was doped with Si or Ge in a relatively high concentration, for example above 1017/cm3, to increase the carrier concentration. In particular, where the active layer has a multiple quantum well structure, the layer is relatively thicker compared to a single quantum well structure, thereby increasing series resistance, in turn increasing a driving voltage and therefore has significantly deteriorated electrical and optical characteristics thus requiring higher doping concentration.
As described above, the scheme of Si or Ge doping necessarily requires an excessive amount of impurities doping, in order to effectively increase the light-gain and thus may present a factor causing crystallinity deterioration. Further, due to a band-tail phenomenon, an oscillating light shifts to a long wavelength region and thus it is difficult to design a light-emitting device with a desired short wavelength.
For these reasons, there remains a need for improving the active layer capable of providing an effect of increasing carrier concentration while reducing crystal defects in the related art.